As shown in FIG. 7, tracer control usually involves detecting axial displacements .epsilon..sub.x, .epsilon..sub.y, .epsilon..sub.z of a stylus 1, which is in contact with a model, by means of a tracer head 2, computing a resultant displacement .epsilon. ##EQU1## obtained by combining the axial displacements by means of a mixing circuit 3, obtaining an error .sub..DELTA. .epsilon. between the resultant displacement .epsilon. and a reference displacement .epsilon..sub.o by an adder circuit 4, generating a normal-direction velocity signal V.sub.N in a velocity signal generator 5 based on the characteristic .sub..DELTA. .epsilon.-V.sub.N, and generating a tangent-direction velocity signal V.sub.T in a velocity signal generator 5 based on the characteristic .sub..DELTA. .epsilon.-V.sub.T. Meanwhile, a tracing direction arithmetic circuit 7 generates cos.theta. and sin.theta. in a tracing direction .theta. using axial displacements .epsilon..sub.1, .epsilon..sub.2 lying in a tracer control plane and outputted by a changeover circuit 8, and an axial velocity signal generator 9 generates axial velocities V.sub.1, V.sub.2 in the tracer control plane using V.sub.N, V.sub.t, cos.theta. and sin .theta., thereby moving the stylus 1 along the model at these velocities. Tracer control is executed by subsequently repeating similar processing.
In a case where a model MDL possesses a steep incline, such as a hollow HL having vertical walls, as shown in FIG. 8, the foregoing tracer control method is such that when the stylus 1 is moved along a vertical wall portion SL, a corner portion 2a of the tracer head 2 is obstructed by the model MDL and tracing cannot be performed.
Accordingly, an object of the invention is to provide a tracer control apparatus which enables a tracer head to trace a model without impedement even if the model profile has projections and cavities of a kind that obstruct the tracer head.
Another object of the invention is to provide a tracer control apparatus which enables a model to be traced while the tracer head is continuously rotated and arranged so as not to be obstructed by the model.